When mounting electronic components on a circuit substrate, in assembling an electronic circuit, a technique is used which comprises bonding a copper foil to the surface of a substrate of an insulating material, as a circuit substrate, forming a preset interconnect pattern by etching, mounting electronic components thereon, and soldering electrodes or terminals for the electronic components to lands for connection of the interconnect pattern of the circuit substrate. For connecting the electrodes of these electronic components to the lands for connection of the circuit substrate, cream solder is used.
That is, cream solder is printed and deposited at preset positions on the surface of the circuit substrate, provided with electrical interconnections, using the technique of screen printing. The electronic components are mounted in position thereon, and the circuit substrate, having the electronic components loaded thereon, is introduced into a reflow furnace. The cream solder is melted by heat in the reflow furnace, whereby the electrodes of the electronic components are soldered to the lands for connection of the circuit substrate. Thus, for the manufacture of the electronic circuit, it is necessary to provide a screen printing apparatus for applying the cream solder on the circuit substrate by the screen printing technique.
An example of such a screen printing apparatus will now be explained with reference to FIGS. 1 and 2. A conventional screen printing apparatus 100 includes a squeegee assembly 110, in which a squeegee 104 is mounted to a squeegee holder 101 by a squeegee retainer 106, and in which a solder leakage guide 102 is mounted on both ends of the squeegee to the squeegee holder 101. In printing a cream solder 105, a printing substrate 107 is contacted with the back surface of a screen 103 and subsequently the squeegee assembly 110 is caused to descend to compress the squeegee 104 against the screen 103. The squeegee assembly is actuated along the direction of printing to cause the squeegee 104 to slide on the screen 103, in order to print the cream solder 105, lying on the screen 103, onto the screen 103. The cream solder 105 is charged in a printing opening area in the screen 103 and printed on the printing substrate 107. It is noted that the printing opening area is an opening formed in the screen for printing the cream solder 105 on the printing substrate 107.
Since the cream solder 105 is high in viscosity, it becomes twisted between the screen 103 and the squeegee 104, as the squeegee 104 sweeps during the printing. The cream solder flows in a direction along arrow G in FIG. 1, perpendicular to the moving direction, as the cream solder is rolled along the printing direction on the printing substrate 107, with the cream solder then leaking out to outside the width of the squeegee, along the direction of an arrow H in FIG. 1, that is, to outside the printing area. The solder leakage guide 102 is provided for preventing the cream solder 105 from leaking to outside the printing area. Other measures for combating the leakage are taken, such as providing a mechanism for raking the cream solder towards the inner side on both ends of the squeegee, for reducing the amount of the cream solder 105 extruded and discharged from both ends of the squeegee during the squeegee driving, or providing an enlarged portion extending laterally from the squeegee. Examples of these measures for preventing leakage of the cream solder are disclosed in the JP Patent Laid-Open Publication JP-Hei-8-39766 (Patent publication 1) and the JP Patent Laid-Open Publication JP-2000-136678 (Patent publication 2).
As means mounted to both ends of the squeegee for prohibiting solder leakage there are known a guide for contacting the solder leakage guide 102 to the screen 103 and a guide for providing a certain clearance between the solder leakage guide 102 and the screen 103. However, as the number of printed paper sheets is increased, it can hardly be avoided that the cream solder 105 becomes twisted between the screen 103 and the squeegee 104 and extruded to outside the squeegee to leak out from the solder leakage guide 102.
Moreover, with the technique disclosed in Patent Publication 1, the squeegee itself becomes complex in structure to raise the cost. In addition, with this technique, there is a risk that, when the squeegee 104 is uplifted on the termination of printing, the solder becomes affixed, as at 108, to the inner lateral side of the solder leakage guide 102, as shown in FIG. 3, this affixed solder 108 being then clinched between the solder leakage guide 102 and the screen 103 to leak to outside the printing area, as leakage solder 109, when the squeegee 104 is lowered for the next printing. The affixed solder 108, affixed to the solder leakage guide 102, and the leakage solder 109, leaked from the guide, become oxidized and unusable with the lapse of time and hence, are discarded. This discarded solder leads to a raised production cost and a wastage of resources, while raising significant pollution problems if the processing for disposal is taken into account.